Semiconductor wafers have the potential to fracture during processing. Semiconductor wafers include but are not limited to the silicon wafers commonly used as substrates for making photovoltaic devices. Wafer breakage during processing is costly, in part because of lost substrate materials, but more importantly because of the time and additional processing materials lost prior to wafer breakage.
One of the strategies for lowering the cost of silicon-based photovoltaic (PV) energy is to use thinner wafers for solar cell fabrication. This strategy reduces the wafer cost and increases solar cell efficiency, provided appropriate cell design and processing techniques are employed. Although the concept of reducing wafer thickness is quite straightforward, it is difficult to implement in manufacturing. The experience in the industry is that even with wafers of typical thickness, for example, wafers having a nominal thickness of about 250 μm, the breakage experienced during solar cell fabrication are quite high. The estimated fraction of wafers that break during cell fabrication and module encapsulation ranges between 5% and 10%. Additional reductions in wafer thickness have been found to further decrease the final product yield to unacceptable values because of breakage.
The yield loss due to wafer breakage has a considerable influence on the economies of producing solar cells. In particular, because the loss in revenue associated with wafer breakage increases as the cell fabrication process progresses, it is desirable to exclude those wafers that may break during cell processing before those wafers enter the fabrication lines. Identifying the sources of wafer breakage, understanding wafer breakage mechanisms, and developing methods of detecting and separating those wafers that are susceptible to breakage is of value, especially at early stages of solar cell fabrication.
Wafer breakage is not a major issue in other semiconductor industries which also uses silicon wafers. The computer semiconductor industry utilizes specific wafer preparation and pre-processing steps designed to minimize wafer breakage. These preventive measures add significant costs to a raw wafer however. In the photovoltaic industry it is not feasible to adopt these preventive measures due to the high cost associated with known semiconductor breakage prevention procedures. Thus, the excessive breakage of wafers experienced in the photovoltaic industry is exacerbated when compared to other semiconductor industries by relatively inadequate wafer preparation, inexpensive wafer handling techniques, and low-cost device processing methods, all of which are aimed at minimizing the cost of the resulting solar cell. In particular, relatively incomplete wafer preparation in the photovoltaic industry leaves defects such as microcracks at the surfaces and the edges of the wafers, which in turn lead to wafer breakage during cell fabrication as discussed herein.
Optical methods are sometimes used in the industry to detect flaws in wafers. For example, methods and apparatus are known for detecting a breakage producing flaw on the edge of a semiconductor wafer using an optical system, which illuminates the edge and measures the scattered radiation with optical detectors, microscopes, or other inspection devices.
Optical techniques however, are most well suited to detect and identify defects or flaws in semiconductor wafers which have polished surfaces. These techniques are difficult to use on photovoltaic wafers because these wafers typically have surfaces with relatively large roughness. The surface roughness “hides” such flaws and microcracks, making it difficult to recognize their presence by optical techniques. Furthermore, even if the detection of microcracks is accomplished, it is not straightforward to predict if the wafer will break during cell or module fabrication. Thus, it would be a significant contribution to the art to provide an effective non-optical method of rapidly screening wafers, which have defects that can result in the wafer breakage during device fabrication.
Commonly owned Patent Application PCT/US06/29765 entitled “Screening of Silicon Wafers Used in Photovoltaics” discloses a system and apparatus for the non-optical screening of wafers used in photovoltaics. The disclosure of the PCT/US06/29765 application is incorporated herein by reference for all matters disclosed therein. The methods and apparatus disclosed in the PCT/US06/29765 application are not however optimized for screening wafers at a rate suitable for the commercial production of photovoltaic devices.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.